Persons skilled in the art of recovering oil or petroleum from subterranean formations ordinarily employ socalled primary recovery techniques first, so long as oil may be recovered under acceptable economic conditions thereby. Once primary production is no longer economically feasible, some form of supplemental or enhanced recovery is applied to the subterranean formation. One of the earliest used and most popular forms of enhanced recovery is water injection, in which either fresh water or brine is injected into the subterranean formation to displace or push the residual oil through the formation toward a spaced-apart production well, from which it is recovered to the surface of the earth. Since the viscosity of the oil present in the subterranean formation is usually higher than the viscosity of water or other aqueous fluids injected into the formation, there is a strong tendency for the more mobile (less viscous) aqueous fluid to bypass a substantial portion of the oil. This is sometimes referred to in the literature as viscous fingering. The result is that only a portion of the residual oil is displaced by the aqueous fluid.
This problem has been recognized by persons skilled in the art of oil recovery, and various literature references describe methos for increasing the ability of the flooding medium to displace residual oil. It is well known in the art of oil recovery and described in the literature pertaining thereto that incorporation of sufficient amount of certain polymeric materials in the aqueous flooding medium to increase the viscosity thereof to a value more nearly equal to or greater than the viscosity of the oil, reduces or eliminates the tendency for the injected aqueous fluid to bypass or finger through the residual oil in the formation. Many substances have been disclosed in the literature for incorporation in the flooding medium for the purpose of increasing the viscosity of the injected fluid. U.S. Pat. No. 2,827,964 and U.S. Pat. No. 3,039,529 describe the use of high molecular weight, partially hydrolyzed polyacrylamides as thickening agents for aqueous fluids employed in oil recovery operations. U.S. Pat. No. 3,581,824 describes the use of heteropolysaccharides produced by fermentation of carbohydrates by bacteria of the genus Xanthomonas for the same purpose.
It is important to recognize the difference between the effect achieved by injecting a viscous, hydrophilic polymer-containing fluid into a formation as contrasted to injecting an aqueous fluid containing a surface active agent, i.e., a surfactant. The surfactant-containing fluid decreases the interfacial tension between the residual oil and the flooding medium in the flow channels through which the fluid passes, and will therefore reduce the residual oil in the portion of the formation contacted by the injected surfactant fluid. A fluid containing any of the hydrophilic polymers normally used for viscous flooding oil recovery methods does not reduce the interfacial tension between residual oil and the injected aqueous medium, and so does not reduce the oil saturation in the flow channels through which it passes. The purpose for using a hydrophilic polymer-containing fluid is to increase the number of flow channnels contacted by the injected fluid, or to improve the volumetric sweep efficiency of the oil recovery method. It is common practice to employ both a surfactant solution and a viscous, hydrophilic polymer-containing fluid in an optimum state-of-the-art chemical flooding process, although either may be used alone without the other.
Aqueous fluids containing suffienct hydrophilic polymer to increase the viscosity thereof to a value equal to or greater than the oil viscosity for the purpose of increasing the volumetric sweep efficiency, are commonly referred to in the art as mobility control or mobility buffer fluids. The ability of the various classes of polymers employed in mobility control fluid to produce the desired increase in the viscosity of the injected fluid depends on various factors including the salinity of the aqueous fluid present in the formation, the physical and chemical characteristics of the formation, and the nature of the residual oil.
It is recognized by persons skilled in the art of enhanced oil recovery processes employing mobility control fluids, that numerous problems are encountered in the use of these fluids. Injectivity problems are sometimes encountered due to improper hydration of the polymer, bacterial growth and other contaminants.
Another important property of an aqueous mobility control fluid relating to the flow resistance of the polymer fluid through a porous medium such as a permeable, subterranean oil-containing earth formation is recognized and a "screen factor" has been defined, which relates to the ability of the fluid to flow under such conditions. The screen factor is a measure of the viscoelastic behavior of the polymer fluid.
Another serious problem which has been recognized as occuring in the use of all of the hydrophilic polymers described in the literature for use in mobility control fluids, is bacterial degradation of the polymer contained in the fluid, which causes loss in fluid viscosity. It is not unusual for fluids injected into subterranean earth formations for oil recovery purposes to remain in the formation for many months or even years, and so the fluid properties will be adversely affected even though the rate of bacterial decomposition of the polymer is relatively slow. Many methods have been described in the literature for reducing the problem associated with bacterial decomposition of hydrophilic polymers, but most which have been described heretofore are either of limited effectiveness or are prohibitively expensive.